For Hearing Instrument (HI) applications (as well as other types of applications such as personal computers and cellular phones), it is typically desirable to provide a highly linear buffered signal from the microphone transducer while maintaining low static power consumption and a minimized input referred noise characteristic. The transducer can be either a subminiature electret or MEMS based transducer with a package footprint that is also minimized versus sensitivity and noise performance. The sensitivity and noise characteristics, and audio band distortion relate to the audio quality of the microphone while distortion from ultrasonic signals can produce audible artifacts through intermodulation of fundamental tones caused by multiple ultrasonic sources or Doppler Effect type frequency shifting. The second order intermodulation product caused by multiple sources in the ultrasonic frequency range is of concern since the difference frequency of tones caused by ultrasonic detectors currently available can demodulate into the audio band.
Ideally, the buffered signal is highly linear for all frequencies in the audio band and ultrasonic frequencies that the HI (or other device) is exposed to during the normal usage such as ultrasonic frequencies produced by ultrasonic detector systems, and so forth. Audio inputs and ultrasonic interferences can be very large amplitudes and the need to drive relatively large ac or dc loads is usually necessary. These loads can be electronic filters, either low pass, high pass, or both, for shaping the output frequency response or the load can be the input impedance to the HI (or other device) input circuitry. Driving these loads with large amplitudes and/or frequencies can demand very large peak currents which can distort the output signal if the peak current requirements cannot be provided within the linear range of the buffer whether by slew rate limiting of the buffer or large signal excursions from the bias point. Since the first buffer is implemented with transistors which are square law devices, the linear operating range represents a relatively small change in current relative to the bias point that is able to be delivered to the load.
HI (or other) applications also require good noise performance and high sensitivity at low quiescent current, so the challenge is to address ultrasonic interference while maintaining good noise performance and input buffer gain.
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